SEOUL, Feb 12 (Reuters) - North Korea conducted its thirdnuclear test on Tuesday in defiance of U.N. resolutions, drawingcondemnation from around the world, including from its onlymajor ally, China, which summoned the North Korean ambassador toprotest.

Pyongyang said the test was an act of self-defence against"U.S. hostility" and threatened stronger steps if necessary.

The test puts pressure on U.S. President Barack Obama on theday of his State of the Union speech and also puts China in atight spot, since it comes in defiance of Beijing'sadmonishments to North Korea to avoid escalating tensions.

The U.N. Security Council held an emergency meeting at whichits members, including China, "strongly condemned" the test andvowed to start work on appropriate measures in response, thepresident of the council said.

North Korean leader Kim Jong-un, the third of his line torule the country, has presided over two long-range rocketlaunches and a nuclear test during his first year in power,pursuing policies that have propelled his impoverished andmalnourished country closer to becoming a nuclear weapons power.

North Korea said the test had "greater explosive force" thanthose it conducted in 2006 and 2009. Its KCNA news agency saidit had used a "miniaturised" and lighter nuclear device,indicating it had again used plutonium, which is suitable foruse as a missile warhead.

Foreign Minister Yang Jiechi said China was "stronglydissatisfied and resolutely opposed" to the test and urged NorthKorea to "stop any rhetoric or acts that could worsen situationsand return to the right course of dialogue and consultation assoon as possible".

Analysts said the test was a major embarrassment to China,which is a permanent member of the Security Council and NorthKorea's sole major economic and diplomatic ally.

Obama called the test a "highly provocative act" that hurtregional stability.

"The danger posed by North Korea's threatening activitieswarrants further swift and credible action by the internationalcommunity. The United States will also continue to take stepsnecessary to defend ourselves and our allies," Obama said.

U.S. Ambassador to the United Nations Susan Rice said Washington and its allies intended to "augment the sanctionsregime" already in place due to Pyongyang's previous atomictests. North Korea is already one of the most heavily sanctionedstates in the world and has few external economic links that canbe targeted.

Japanese Prime Minister Shinzo Abe said the test was a"grave threat" that could not be tolerated.

South Korea, still technically at war with North Korea aftera 1950-53 civil war ended in a mere truce, also denounced thetest. Obama spoke to South Korean President Lee Myung-bak onTuesday and told him the United States "remains steadfast in itsdefence commitments" to Korea, the White House said.

MAXIMUM RESTRAINT

North Korea's Foreign Ministry said the test was "only thefirst response we took with maximum restraint".

"If the United States continues to come out with hostilityand complicates the situation, we will be forced to takestronger, second and third responses in consecutive steps," itsaid in a statement carried by the official KCNA news agency.

North Korea often threatens the United States and its"puppet", South Korea, with destruction in colourful terms.

North Korea told the U.N. disarmament forum in Geneva thatit would never bow to resolutions on its nuclear programme andthat prospects were "gloomy" for the denuclearisation of thedivided Korean peninsula because of a "hostile" U.S. policy.

Suzanne DiMaggio, an analyst at the Asia Society in NewYork, said North Korea had embarrassed China with the test."China's inability to dissuade North Korea from carrying throughwith this third nuclear test reveals Beijing's limited influenceover Pyongyang's actions in unusually stark terms," she said.

Mark Fitzpatrick of the International Institute forStrategic Studies think tank, said: "The test is hugelyinsulting to China, which now can be expected to follow throughwith threats to impose sanctions."

The magnitude of the explosion was roughly twice that of the 2009 test, according to the Vienna-based ComprehensiveNuclear-Test Ban Treaty Organization. The U.S. Geological Surveysaid that a seismic event measuring 5.1 magnitude had occurred.